1. Field of the Invention
This invention relates generally to methods and apparatus for desorption and ionization of analytes for the purpose of subsequent scientific analysis by such methods, for example, as mass spectrometry or biosensors. More specifically, this invention relates to the field of mass spectrometry, especially to the type of matrix-assisted laser desorption/ionization, time-of-flight mass spectrometry used to analyze macromolecules, such as proteins or biomolecules.
2. Description of the Prior Art
Generally, analysis by mass spectrometry involves the vaporization and ionization of a small sample of material, using a high energy source, such as a laser, including a laser beam. The material is vaporized from the surface of a probe tip by the laser beam, and in the process, some of the individual molecules are ionized by the gain of a proton. The positively charged ionized molecules are then accelerated through a short high voltage field and let fly into a high vacuum chamber, at the far end of which they strike a sensitive detector surface. Since the time of flight is a function of the mass of the ionized molecule, the elapsed time between ionization and impact can be used to determine the molecule""s mass which, in turn, can be used to identify the presence or absence of known molecules of specific mass.
All known prior art procedures which present proteins or other large biomolecules on a probe tip for laser desorption/ionization time-of-flight mass spectrometry rely on a crystalline solid mixture of the protein or other analyte molecule in a large excess of acidic matrix material deposited on the bare surface of a metallic probe tip. (The sample probe tip typically is metallic, either stainless steel, nickel plated material or platinum). Immobilizing the analyte in such a matrix was thought to be necessary in order to prevent the destruction of analyte molecules by the laser beam. The laser beam strikes the mixture on the probe tip and its energy is used to vaporize a small portion of the matrix material along with some of the embedded analyte molecules. Without the matrix, the analyte molecules are easily fragmented by the laser energy, so that the mass, and identity, of the original macromolecule is very difficult to determine.
This prior art procedure has several limitations which have prevented its adaptation to automated protein or other macrobiological molecules analysis. First, in a very crude sample it is necessary to partially fractionate (or otherwise purify the sample as much as possible) to eliminate the presence of excessive extraneous materials in the matrix/analyte crystalline mixture. The presence of large quantities of components may depress the signal of the targeted analyte. Such purification is time-consuming and expensive and would be very difficult to do in an automated analyzer.
Second, while the amount of analyte material needed for analysis by the prior art method is not large (typically in a picomole range), in some circumstances, such as tests on pediatric patients, analyte fluids are available only in extremely small volumes (microliters) and may be needed for performing several different analyses. Therefore, even the small amount needed for preparation of the analyte/matrix crystalline mixture for a single analysis may be significant. Also, only a tiny fraction (a few thousandths or less) of analyte used in preparing the analyte/matrix mixture for use on the probe tip is actually consumed in the mass spectrometry analysis. Any improvement in the prior art procedure which made it possible to use much less analyte to conduct the test would be highly advantageous in many clinical areas.
Third, the analyte protein, or other macromolecule, used in preparing the analyte matrix for use on the probe tip is not suitable for any subsequent chemical tests or procedures because it is bound up in the matrix material. Also, all of the matrix material used to date is strongly acidic, so that it would affect many chemical reactions which might be attempted on the mixture in order to modify the analyte molecules for subsequent examination. Any improvement in the procedure which made it possible to conduct subsequent chemical modifications or reactions on the analyte molecules, without removing them from the matrix or the probe tip, would be of enormous benefit to researchers and clinicians.
Additional limitations in the prior art included problems with matrix use such as:
(1) formation of analyte-matrix complex (referred to as xe2x80x9cmatrix adductxe2x80x9d which interferes with the accuracy of analyte measurement;
(2) inability to wash away contaminants present in analyte or matrix (e.g., other proteins or salts);
(3) formation of analyte-salt ion adducts;
(4) less than optimum solubility of analyte in matrix;
(5) signal (molecular ion) suppression xe2x80x9cpoisoningxe2x80x9d due to simultaneous presence of multiple components; and
(6) selective analyte desorption/ionization.
There are a number of problems and limitations with the prior art methods. Prior investigators, including Karas and Hillenkamp have reported a variety of techniques for analyte detection using mass spectroscopy, but these techniques suffered because of inherent limitations in sensitivity and selectivity of the techniques, specifically including limitations in detection of analytes in low volume, undifferentiated samples. The xe2x80x9cHillenkamp-Karasxe2x80x9d articles that pertain to this field of invention are:
1. Hillenkamp, xe2x80x9cLaser Desorption Mass Spectrometry: Mechanisms, Techniques and Applicationsxe2x80x9d; Bordeaux Mass Spectrometry Conference Report, 1988, pages 354-362.
2. Karas and Hillenkamp, xe2x80x9cUltraviolet Laser Desorption of Proteins Up to 120,000 Daltonsxe2x80x9d, Bordeaux Mass Spectrometry Conference Report, 1988, pages 416, 417.
3. Karas and Hillenkamp, xe2x80x9cLaser Desorption Ionization of Proteins With Molecular Masses Exceeding 10,000 Daltonsxe2x80x9d, Analytical Chemistry, 60. 2299-2301, July 1988.
4. Karas, Ingendoh, Bahr and Hillenkamp, xe2x80x9cUV-Laser Desorption/Ionization Mass Spectrometry of Femtomol Amounts of Large Proteinsxe2x80x9d, Biomed. Environ. Mass Spectrum (cross-linked).
The use of laser beams in time-of-flight mass spectrometers is shown, for example, in U.S. Pat. Nos. 4,694,167; 4,686,366, 4,295,046, and 5,045,694, incorporated by reference.
The first successful molecular mass measurements of intact peptides and small proteins (only up to about 15 kDa) by any form of mass spectrometry were made by bombarding surfaces with high energy particles (plasma desorption and fast atom bombardment mass spectrometry); this breakthrough came in 1981 and 1982. Improvements came in 1985 and 1986, however, yield (signal intensities), sensitivity, precision, and mass accuracy remained relatively low. Higher molecular mass proteins (about 20 to 25 kDa) were not observed except on rare occasions; proteins representing average molecular weights (approximately 70 kDa) were not ever observed with these methods. Thus, evaluation of most proteins by mass spectrometry remains unrealized.
In 1988, Hillenkamp and his coworkers used UV laser desorption time-of-flight mass spectrometry and discovered that when proteins of relatively high molecular mass were deposited on the probe tip in the presence of a very large molar excess of an acidic, UV absorbing chemical matrix (nicotinic acid) they could be desorbed in the intact state. This new technique is called matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Note that laser desorption time-of-flight mass spectrometry (without the chemical matrix) had been around for some time, however, there was little or no success determining the molecular weights of large intact biopolymers such as proteins and nucleic acids because they were fragmented (destroyed) upon desorption. Thus, prior to the introduction of a chemical matrix, laser desorption mass spectrometry was essentially useless for the detection of specific changes in the mass of intact macromolecules (see below). Note that the random formation of matrix crystals and the random inclusion of analyte molecules in the solid solution is prior art.
The primary object of the invention is to provide improved methods, materials composition and apparatus for coupled adsorption, desorption and ionization of multiple or selected analytes into the gas (vapor) phase, preferably for use in conjunction with mass spectrometry of biomolecules and other macromolecules, as well as by means of analytic detection other than mass spectrometry. The invention incudes a flexible variety of options for presenting surfaces with attached energy absorbing molecules, defined reaction sites, and affinity reagents for the capture,e transfer, and/or the desorption of analytes before and after a sseries of chemical, physical, and/or enzymatic modifications performed in situ.
Another object is to provide such a method and apparatus for affinity-directed detection of analytes, including desorption and ionization of analytes in which the analyte is not dispersed in a matrix solution or crystalline structure but is presented within, on or above an attached surface of energy absorbing xe2x80x9cmatrixxe2x80x9d material through molecular recognition events, in a position where it is accessible and amenable to a wide variety of chemical, physical and biological modification or recognition reactions.
The probe surface with and without bonded energy absorbing molecules, referred to as the sample presenting surface can be composed of a variety of materials, including porous or nonporous materials, with the porous materials providing sponge-like, polymeric, high surface areas for optimized adsorption and presentation of analyte.
These surface materials can be substituted (at varying densities) with chemically bonded (covalent or noncovalent) affinity adsorption reagents and/or chemically bonded (i.e., immobilized) energy absorbing molecules (bound xe2x80x9cmatrixxe2x80x9d molecules). The geometry of the sample presenting surface can be varied (i.e., size, texture, flexibility, thickness, etc.) to suit the need (e.g., insertion into a living organism through spaces of predetermined sizes) of the experiment (assay).
Another object is to provide such a method and apparatus in which the analyte material is chemically bound or physically adhered to a substrate forming a probe tip or other sample presenting surface.
A further object is to provide means for the modification of probe or sample presenting surfaces with energy-absorbing molecules to enable the successful desorption of analyte molecules without the addition of exogenous matrix molecules as in prior art.
A further object is to provide the appropriate density of energy-absorbing molecules bonded (covalently or noncovalently) in a variety of geometries such that mono layers and multiple layers of attached energy-absorbing molecules can be used to facilitate the desorption of analyte molecules of varying masses. The optimum ratio of adsorbed or bonded energy-absorbing molecules to analyte varies with the mass of the analyte to be detected. A further object is to modify the sample presenting surface with such energy-absorbing molecules where the composition of the probe or sample presenting surface is other than the metal or metallic surfaces as described in prior art. Separate from the chemical and/or physical modification of the probe surface with energy absorbing molecules is the modification of these surfaces with affinity reagents, both chemical and/or biological, for the specific purpose of capturing (adsorbing) specific analyte molecules or classes of analyte molecules for the subsequent preparation, modification, and successful desorption of said analyte molecules.
A further object is to provide all combinations of surfaces modified with energy-absorbing molecules and/or affinity-directed analyte capture devices to enable the selective and/or nonselective adsorption of analytes and the subsequent desorption either with or without requiring the subsequent addition of additional matrix molecules. It is important to note that the surfaces modified with affinity reagents for the capture of analytes are more useful than the underivitized sample surfaces described in prior art even when the deposition of energy-absorbing molecules (that is matrix) is as described in prior art. Because of the advantages in the ability to remove contaminating substances from the adsorbed analyte molecules and because of the ability to modify adsorbed analyte molecules without (or before) added matrix.
A further object is to provide such a method and apparatus in which the substrate forming the probe tip or other sample presenting surface is derivatized with one or more affinity reagents (a variety of densities and degrees of amplification) for selective bonding with predetermined analytes or classes of analytes.
A further object is to provide methods and apparatus for using probe tips having surfaces derivatized with affinity reagents and containing laser desorption matrix material (chemically bonded to surface or not) which may be used to isolate target analyte materials from undifferentiated biological samples such as blood, tears, urine, saliva, gastrointestinal fluids, spinal fluid, amniotic fluid, bone marrow, bacteria, viruses, cells in culture, biopsy tissue, plant tissue or fluids, insect tissue or fluids, etc.
Because of the new and preferred method for presentation and desorption of selected analytes, a further object is to use analyte detection methods other than the generic electron multipliers typically used in mass spectrometric devices. This would include but would not be limited to detection films/plates for the qualitative or quantitative evaluation of fluorescent or radio-labeled analytes or analyte molecule complexes.
A further object is to provide such a system in which the affinity reagent chemically bonds or biologically adheres to the target analyte or class of analytes.
A further object is to use existing and new solid phase affinity reagents (e.g., small diameter porous or nonporous beads of crosslineated polymer with attached molecular capture devices) designed for the (1) capture (adsorption of one or more analytes, (2) the preparation of these captured analytes (e.g., washing with H2O or other buffered or nonbuffered solutions to remove contaminants such as salts, multiple cycles of washing, such as with polar organic solvent, detergent-dissolving solvent, dilute acid, dilute base or urea), and (3) most importantly, the direct transfer of these captured and prepared analytes to the probe surface for subsequent analyte desorption (for detection, quantification and/or mass analysis).
A further object is to provide such a system in which the predetermined analytes are individual biomolecules or other macromolecules or combinations of adjoined molecules (i.e., complexes).
A still further object is to provide such a method and apparatus in which the matrix materials used are not strongly acidic, as in prior art matrices, but are chemically modified into the slightly acidic, neutral pH or strongly basic range of pH.
A further object is to provide such a system in which the matrix material has a pH above 6.0.
A still further object is to provide a method and apparatus for desorption and ionization of analytes in which unused portion of the analytes contained on the presenting surface remain chemically accessible, so that a series of chemical and/or enzymatic or other treatments (e.g., discovery of analyte-associated molecules by molecular recognition) of the analyte may be conducted on the probe tip or other presenting surface, in situ, followed by sequential analyses of the modified analyte by mass spectrometry. In one case (i.e., repetitive sequential analyses) the analyte is adsorbed to the sample presenting surface and can be treated (modified in situ after the excess free matrix is removed (i.e., washed away). Matrix can be added back before analysis by mass spectrometry. Using this procedure, an analyte can be repeatedly tested for a variety of components by removing one matrix, modifying the analyte sample, re-applying the same or different matrix, analyzing the sample, etc.
A further object is to provide a method and apparatus for the combined chemical and/or enzymatic modifications of target analytes for the purpose of elucidating primary, secondary, tertiary, or quaternary structure of the analyte and its components.
A still further object is to provide such a method and apparatus in which the probe tips or other sample presenting surfaces are formed of a variety of materials, including electrically insulating materials (porous and nonporous), flexible or nonrigid materials, optically transparent materials (e.g., glass, including glass of varying densities, thicknesses, colors and with varying refractive indices), as well as less reactive, more biocompatible materials (e.g., biopolymers such as agarose, dextran, cellulose, starches, peptides, and fragments of proteins and of nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). These surfaces can be chemically modified by the attachment of energy-absorbing molecules and/or affinity directed analyte capture molecules.
Another object is to provide a method and apparatus for desorption and ionization of analyte materials in which cations other than protons (H+) are utilized for ionization of analyte macromolecules.
Note that the laser or light source used to convey energy to the probe surface can employ a wavelength(s) that is(are) not fixed but can be varied according to the wavelength absorbed by the matrix (whether the matrix is added in the free form or is chemically bonded to the probe (sample presenting) surface). For this procedure, a variety of wavelengths (10 or more) defined by absorbance of matrix or energy absorbing surface can be utilized.
Another object is to provide such a method and apparatus in which the probe tips or other sample presenting surfaces used for laser desorption/ionization time-of-flight mass spectrometry are magnetized and in which the matrix, affinity directed absorption molecules and/or analyte materials are magnetically adhered to such magnetized surface.
A further object is a method and apparatus in which the matrix and/or analyte materials are adhered by any variety of chemical mechanisms to the sample presenting surface.
A further objective is to provide energy-absorbing molecules which have been incorporated into other chemical structures (e.g., chemical or biological polymers) for the deposition (covalent or noncovalent) onto the sample presenting surface in a way that enables repetitious analyte desorption events without interference with chemical and/or enzymatic modifications of the analyte molecule(s) performed in situ.
Another object is to provide sample presenting surfaces in a variety of sizes and configurations (up to 4xe2x80x3xc3x974xe2x80x3) with multiple (up to 10,000 or more) spots (including spots down to  less than 0.001 inch diameter) of affinity reagents arranged in predetermined arrays for the selective adsorption of numerous different analytes (e.g., clinical chemical marker proteins) to enable a wide spectrum sampling of the macromolecular composition of biological samples/fluids.
As shown more fully below, the present invention overcomes limitations and disabilities of the prior art by providing probe tips or sample plates whose surfaces have been derivatized with biospecific affinity reagents which will selectively bind specific groups or types of biomolecules or other analytes out of an undifferentiated sample (such as blood or urine). Appropriate selection of the affinity reagents used to derivatize the probe tip surface therefore makes possible the selection from the undifferentiated sample and binding to the probe tip of the specific types or groups of biological or other macromolecules under investigation, or subsequent examination (e.g., quantification and/or structure elucidation) by mass spectrometry. This has the advantage of achieving both the purification of the analyte sample previously required and the effect of concentrating the analyte. It reduces by a factor of 1,000 to 100,000 the amount of analyte needed for the mass spectrometry examination, since only the macromolecules which attach to the biospecific affinity reagents are removed from the analyte sample, and these can be sequestered on predetermined areas of the probe tips or sample plates that are even less than the laser spot size.
It also has been found that the probe tips used in the process of the invention need not be metal or metal-coated, as with prior art procedures. Research involved in the invention has involved glass and synthetic polymer surfaces such as polystyrene, polypropylene, polyethylene, polycarbonate and other polymers including biopolymers, for the probe tips which have been covalently or noncovalently derivatized for immobilization of specific reagents that will direct the selective adsorption of specific analytes. These surfaces will include immobilized metal ions, immobilized proteins, peptides, enzymes, and inhibitor molecules, immobilized DNA and RNA, immobilized antibodies, immobilized reducing agents, immobilized carbohydrates and lectins, immobilized dyes and immobilized protein surface domains involved in molecular recognition (e.g., dimerization domains and subunits). Some of the chemical and surface structures are as yet unknown.
The preferred probe tip, or sample plate, for selective adsorption/presentation of sample for mass analysis are (1) stainless steel (or other metal) with a synthetic polymer coating (e.g., cross-linked dextran or agarose, nylon, polyethylene, polystyrene) suitable for covalent attachment of specific biomolecules or other nonbiological affinity reagents, (2) glass or ceramic, and/or (3) plastic (synthetic polymer). The chemical structures involved in the selective immobilization of affinity reagents to these probe surfaces will encompass the known variety of oxygen-dependent, carbon-dependent, sulfur-dependent, and/or nitrogen-dependent means of covalent or noncovalent immobilization. The methods and chemical reactions used in producing such surfaces derivatized with biospecific affinity reagents already are known by those skilled in the art. Two features of the invention, however, are (1) the specific size and localization of the derivatized surface with respect to the laser beam and (2) the affinity directed presentation of specific analyte molecules (e.g., macromolecule or biopolymer) at a defined surface density or local concentration required for the efficient detection by laser desorption/ionization time-of-flight mass spectrometry. This can be accomplished by arranging the affinity adsorption xe2x80x9cspotsxe2x80x9d (0.005 to 0.080 inch diameter) on the probe surface in a defined manner (400 to 1,000 spots could be placed on a surface about the size of a glass slide).
An additional discovery involves the fact that pH modified chemical matrices can be used on these surfaces to facilitate desorption/ionization without disruption of conditions necessary for subsequent sample modification. As discussed above, prior art matrix materials used for biomolecular mass spectrometry are acidic. The exact chemical structure of the pH-modified matrices still are unknown. However, by suitable neutralization of the matrix material, it can be made largely passive to subsequent chemical or enzymatic reactions carried out on the analyte molecules presented on the derivatized probe tip surface by the biospecific affinity reagents. This makes possible the carrying out of chemical reactions on the analyte molecules presented on the probe tips. Since only a small fraction of the analyte molecules are used in each desorption/mass spectrometer measurement, a number of sequential chemical and/or enzymatic modifications of the samples, in situ, on the probe tips, and subsequent analysis of the modified samples by mass spectrometry, can be carried out on the same probe tips in order to more accurately determine exactly what molecule is present, or other characteristics or information about the molecule, including its structure.
Finally, even when these matrix molecules are immobilized on a probe tip surface, the analyte deposited on such a surface can be desorbed with a laser beam. This circumvents the contamination of the analyte by the matrix molecules. As a particular feature of the invention, we have shown that some energy absorbing molecules that do not work as xe2x80x9cmatrixxe2x80x9d molecules when added to analytes as a solution of free molecules (as in prior art) do indeed work well to facilitate the desorption of intact analyte molecules after being immobilized.
It seems likely that these improvements in the procedure will enable bioanalytical and medical instrument manufacturers to develop a machine for the automated evaluation of a single protein sample deposited on a surface and modified with numerous chemical and/or enzymatic reactions performed in situ.